CJK Unified Ideographs

The Chinese, Japanese and Korean (CJK) scripts share a common background, collectively known as CJK characters. In the process called Han unification, the common (shared) characters were identified and named "CJK Unified Ideographs." As of Unicode 9.0, Unicode defines a total of 80,388 CJK Unified Ideographs.[1]

The terms ideographs or ideograms may be misleading, since the Chinese script is not strictly a picture writing system.

Historically, Vietnam used Chinese ideographs too, so sometimes the abbreviation "CJKV" is used. This system was replaced by the Latin-based Vietnamese alphabet in the 1920s.

The basic block named CJK Unified Ideographs (4E00–9FFF) contains 20,950 basic Chinese characters in the range U+4E00 through U+9FD5. The block not only includes characters used in the Chinese writing system but also kanji used in the Japanese writing system and hanja, whose use is diminishing in Korea. Many characters in this block are used in all three writing systems, while others are in only one or two of the three. Chinese characters were also used in Vietnam's Nôm script (now obsolete). The first 20,902 characters in the block are arranged according to the Kangxi Dictionary ordering of radicals. In this system the characters written with the fewest strokes are listed first. The remaining characters were added later, and so are not in radical order.

The block is the result of Han unification,[2] which was somewhat controversial in the Far East.[3] Since Chinese, Japanese and Korean characters were coded in the same location, the appearance of a selected glyph could depend on the particular font being used. However, the source separation rule states that characters encoded separately in an earlier character set would remain separate in the new Unicode encoding.[4]

Using variation selectors, it is possible to specify certain variant CJK ideograms within Unicode. The Adobe-Japan1 character set proposal, which actually calls for 14,679 ideographic variation sequences,[5] is an extreme example of the use of variation selectors.[6]

The block named CJK Unified Ideographs Extension B (20000–2A6DF) contains 42,711 characters in the range U+20000 through U+2A6D6 that were added in Unicode 3.1 (2001). These include most of the characters used in the Kangxi Dictionary that are not in the basic CJK Unified Ideographs block, as well as many Nôm characters that were formerly used to write Vietnamese.

The block named CJK Compatibility Ideographs (F900–FAFF) was created to retain round-trip compatibility with other standards. Only twelve of its characters have the "Unified Ideograph" property: U+FA0E, FA0F, FA11, FA13, FA14, FA1F, FA21, FA23, FA24, FA27, FA28 and FA29.[1] None of the other characters in this and other "Compatibility" blocks relate to CJK Unification.

The Ideographic Rapporteur Group (IRG) bears the formal responsibility of developing extensions to the encoded repertoires of unified CJK ideographs. The Unicode Consortium participates in this group as a liaison member of ISO. The characters submitted by the Unicode Technical Committee bear the prefix "UTC". All CJK Unified Ideographs in ISO/IEC10646 are required to have at least one source identifier. Changes to IRG source information, however, can leave a given ideograph without any such sources. In such cases, the ideograph is included in the U-source database to guarantee it has at least one source. Such ideographs are indicated by a source prefix of "UCI" instead of "UTC".[9]

The character U+4039 (䀹) was a unification of two different characters (one with jiā 夾 phonetic and one with shǎn 㚒 phonetic) until Unicode 5.0. However, they were lexically different characters that should not have been unified; they have different pronunciations and different meanings.

The proposal of disunification of U+4039[10] was accepted and the new character is encoded at U+9FC3 (鿃) in Unicode 5.1.

In CJK Unified Ideographs Extension B, hundreds of glyph variants were encoded.[11] In addition to the deliberate encoding of close glyph variants, six exact duplicates (where the same character has inadvertently been encoded twice) and two semi-duplicates (where the CJK-B character represents a de facto disunification of two glyph forms unified in the corresponding BMP character) were encoded by mistake:[12]

U+34A8 㒨 = U+20457 𠑗 : U+20457 is the same as the China-source glyph for U+34A8, but it is significantly different from the Taiwan-source glyph for U+34A8

U+3DB7 㶷 = U+2420E 𤈎 : same glyph shapes

U+8641 虁 = U+27144 𧅄 : U+27144 is the same as the Korean-source glyph for U+8641, but it is significantly different from the China-, Taiwan- and Japan-source glyphs for U+8641

Apart from the six blocks of "Unified Ideographs," Unicode has about a dozen more blocks with not-unified CJK-characters. These are mainly CJK radicals, strokes, punctuation, marks, symbols and compatibility characters. Although some characters have their (decomposable) counterparts in other blocks, the usages can be different.

Four blocks of compatibility characters are included for compatibility with legacy text handling systems and older character sets:

They include forms of characters for vertical text layout and rich text characters that Unicode recommends handling through other means. Therefore their use is discouraged.

Usually, compatibility characters are those that would not have been encoded except for compatibility and round-trip convertibility with other standards. However, the amount of CJK ideographs within any non-Unicode standard is too big to fit into Unicode's CJK Compatibility Ideographs blocks. Instead, code points are assigned when the affected characters are approved by the Unicode Consortium, but have yet to assign any code points within the CJK Unified Ideographs blocks.